The present invention relates in general to substrate manufacturing technologies and in particular to methods and apparatus for tuning a set of plasma processing steps.
In the processing of a substrate, e.g., a semiconductor substrate or a glass panel such as one used in flat panel display manufacturing, plasma is often employed. As part of the processing of a substrate for example, the substrate is divided into a plurality of dies, or rectangular areas, each of which will become an integrated circuit. The substrate is then processed in a series of steps in which materials are selectively removed (etching) and deposited (deposition) in order to form electrical components thereon.
In an exemplary plasma process, a substrate is coated with a thin film of hardened emulsion (i.e., such as a photoresist mask) prior to etching. Areas of the hardened emulsion are then selectively removed, causing components of the underlying layer to become exposed. The substrate is then placed in a plasma processing chamber on a substrate support structure comprising a mono-polar or bi-polar electrode, called a chuck or pedestal. Appropriate plasmas are then sequentially struck to in order to etch various exposed layers on the substrate.
Plasma is generally comprised of partially ionized gas. Because the plasma discharge is RF driven and weakly ionized, electrons in the plasma are not in thermal equilibrium with ions. That is, while the heavier ions efficiently exchange energy by collisions with the background gas (e.g., argon, etc.), electrons absorb the thermal energy. Because electrons have substantially less mass than that of ions, electron thermal velocity is much greater than the ion thermal velocity. This tends to cause the faster moving electrons to be lost to surfaces within the plasma processing system, subsequently creating positively charged ion sheath between the plasma and the surface. Ions that enter the sheath are then accelerated into the surface.
Lower RF frequencies tend to cause plasma ions to cross the sheath in less than one RF cycle, creating large variations in ion energy. Likewise, higher RF frequencies tend to cause plasma ions take several RF cycles to cross the sheath, creating a more consistent set of ion energies. Higher frequency tends to result in lower sheath voltages than when excited by a lower frequency signal at a similar power level.
Referring now to FIG. 1, a simplified diagram of plasma processing system components is shown. Generally, an appropriate set of gases is flowed into chamber 102 through an inlet 108 from gas distribution system 122. These plasma processing gases may be subsequently ionized to form a plasma 110, in order to process (e.g., etch or deposition) exposed areas of substrate 114, such as a semiconductor substrate or a glass pane, positioned with edge ring 115 on an electrostatic chuck 116. In addition, liner 117 provides a barrier between the plasma and the plasma processing chamber, as well as helping to optimize plasma 110 on substrate 114.
Gas distribution system 122 is commonly comprised of compressed gas cylinders 124a-f containing plasma processing gases (e.g., C4F8, C4F6, CHF3, CH2F3, CF4, HBr, CH3F, C2F4, N2, O2, Ar, Xe, He, H2, NH3, SF6, BCl3, Cl2, WF6, etc.). Gas cylinders 124a-f may be further protected by an enclosure 128 that provides local exhaust ventilation. Mass flow controllers 126a-f are commonly a self-contained devices (consisting of a transducer, control valve, and control and signal-processing electronics) commonly used in the semiconductor industry to measure and regulate the mass flow of gas to the plasma processing system. Injector 109 introduces plasma processing gases 124 into chamber 102.
Induction coil 131 is separated from the plasma by a dielectric window 104, and generally induces a time-varying electric current in the plasma processing gases to create plasma 110. The window both protects induction coil from plasma 110, and allows the generated RF field to penetrate into the plasma processing chamber. Further coupled to induction coil 131 at leads 130a-b is matching network 132 that may be further coupled to RF generator 138. Matching network 132 attempts to match the impedance of RF generator 138, which typically operates at 13.56 MHz and 50 ohms, to that of the plasma 110.
Generally, some type of cooling system is coupled to the chuck in order to achieve thermal equilibrium once the plasma is ignited. The cooling system itself is usually comprised of a chiller that pumps a coolant through cavities in within the chuck, and helium gas pumped between the chuck and the substrate. In addition to removing the generated heat, the helium gas also allows the cooling system to rapidly control heat dissipation. That is, increasing helium pressure subsequently also increases the heat transfer rate. Most plasma processing systems are also controlled by sophisticated computers comprising operating software programs. In a typical operating environment, manufacturing process parameters (e.g., voltage, gas flow mix, gas flow rate, pressure, etc.) are generally configured for a particular plasma processing system and a specific recipe.
In general, there are three types of etch processes used to etch the various layers on the substrate: pure chemical etch, pure physical etch, and reactive ion etch.
Pure chemical etching generally involves no physical bombardment, but rather a chemical interaction of neutral molecules (neutrals) with materials on the substrate (e.g., Al, etc.). Subsequently, the chemical reaction rate could be very high or very low, depending on the process. For example, fluorine-based molecules tend to chemically interact with dielectric materials on the substrate, wherein oxygen-based molecules tend to chemically interact with organic materials on the substrate, such as photoresist.
Pure ion etching, often called sputtering, is used to dislodge material from the substrate (e.g., oxide, etc.). Commonly an inert gas, such as Argon, is ionized in a plasma and subsequently accelerate toward a negatively charged substrate. Pure ion etching is both anisotropic (i.e., principally in one direction) and non-selective. That is, selectivity to a particular material tends to be very poor, since sputtering rate of most materials are similar. In addition, the etch rate of the pure ion etching is commonly low, depending generally on the flux and energy of the ion bombardment.
Reactive ion etch (RIE), also called ion-enhanced etching, combines both chemical and ion processes in order to remove material from the substrate (e.g., photoresist, BARC, TiN, Oxide, etc.). Generally ions in the plasma enhance a chemical process by striking the surface of the substrate, and subsequently breaking the chemical bonds of the atoms on the surface in order to make them more susceptible to reacting with the molecules of the chemical process. Since ion etching is mainly perpendicular, while the chemical etching is both perpendicular and vertical, the perpendicular etch rate tends to be much faster than in then horizontal direction. In addition, RIE tends to have an anisotropic profile.
However, one problem that has been encountered with both pure chemical etching and RIE etching has been a non-uniform etch rate. Etch rate is generally the measure of how fast material is removed in the etch process. It is generally calculated by measuring the thickness before and after the etch process and dividing the thickness difference by the etch time:
                              ETCH          ⁢                                          ⁢          RATE                =                                            THICKNESS              ⁢                                                          ⁢              BEFORE              ⁢                                                          ⁢              ETCH                        -                          THICKNESS              ⁢                                                          ⁢              AFTER              ⁢                                                          ⁢              ETCH                                            ETCH            ⁢                                                  ⁢            TIME                                              FIG        .                                  ⁢        1            
Uniformity is generally measured with substrate thickness mapping by measuring the thickness at certain points before and after the etch process, and calculating the etch rates at these points. The mean value (or average value) of the measurement is:
                              x          _                =                                            x                              1                +                                      ⁢                          x                              2                +                                      ⁢                          x                              3                +                …                +                                      ⁢                          x              N                                N                                    FIG        .                                  ⁢        2            Where x is the etch rate at a specific point, on the substrate, and N is the total number of points.
The max-minus-min nonuniformity is defined as:
                              NU          M                =                              (                                          x                max                            -                              x                min                                      )                                2            ⁢                          x              _                                                          FIG        .                                  ⁢        3            
For example, one area of the substrate may be etched at a faster rate than another area. In general, a non-uniform etch may cause undercutting in the side walls of a trench. Typically, undercutting reduces the thickness of the conducting line or in some cases causes line breakage, which may lead to device failure. Still further, non-uniformity etching generally adds time to the etching process, which reduces processing throughput.
This problem is further aggravated for different types of sequential etch process chemistries. For example, often in a chemical or RIE etch process, the etch rate is typically higher at the edge of the substrate where the local etch rate may be dominated by either chemical reactions at the surface, or by limited etchant transport to the substrate surface.
Referring now to FIGS. 2A-C, a set of simplified figures are shown of a plasma comprising ions and neutrals over a substrate. It is often advantageous in the processing of substrates to etch as many substrate layers as possible during a single processing session (i.e., in-situ). For example, in-situ processing tends to minimize the handling of each substrate, and hence to improve yield, to improve the overall production throughput, and to help minimize the amount of plasma processing chambers required. It would therefore be beneficial to configure plasma processing chamber such that the density of the neutrals and that of the ion are substantially uniform among the various types of plasma chemistries, since a substantially uniform plasma density generally produces a substantially uniform etch. FIG. 2A shows a simplified diagram of a plasma processing chamber, in which the neutral density 110a and the ion density 110b are substantially uniform across the surface of substrate 114.
In addition, the portion of the plasma that extends beyond the edge of the substrate may create a larger volume of neutrals available to etch the edge of the substrate as opposed to the center. FIG. 2B shows a simplified diagram of a plasma processing chamber, in which the neutral density 110a is not substantially uniform, subsequently producing a non-uniform etch profile across the surface of substrate 114.
Another solution may be to narrow the diameter of the plasma chamber in order to substantially equalize the amount of neutrals over the substrate. However, for processes that substantially use ions, narrowing the chamber would also cause more ions to be consumed by collisions with the chamber walls. This would tend decrease the ion concentrations, and hence the etch rate, at the edge of the substrate. FIG. 2C shows a simplified diagram of a plasma processing chamber, in which ion density 110b is not substantially uniform, subsequently producing a non-uniform etch profile across the surface of substrate 114.
In view of the foregoing, there are desired methods and apparatus for tuning a set of plasma processing steps.